Humidification in the long-term ventilated patient: a systematic review

Bench S

CRD summary

This review assessed heated humidifiers and heat moisture exchange (HME) devices in intubated intensive care patients being ventilated for more than 48 hours. The author concluded that there was no difference in tracheal tube occlusion between devices, but that HME reduced ventilator-associated pneumonia compared with heated water humidification. The two identified studies provide limited evidence.

Authors' objectives

To compare the effectiveness of heated humidifiers with heat moisture exchange (HME) devices in intubated intensive care patients, who have been ventilated for more than 48 hours, in preventing tracheal tube obstruction and ventilator-associated pneumonia (VAP).

Searching

The Cochrane Library, MEDLINE (from 1993 to present), CINAHL (from 1984 to present) and EMBASE (from 1980 to present) were searched for studies published in the English language; the search strategy was described. Pre-1995 studies were excluded. The studies had to be either available online or from the Kings College London group of libraries.

Study selection

Study designs of evaluations included in the review

Randomised controlled trials (RCTs), which were adequately powered where the results were statistically non significant, were eligible for inclusion. The studies had to be truly randomised and present information on ventilator circuit changes, filter changes, suction episodes, instillation of saline and physiotherapy sessions, so that it could be determined that the groups were treated equally apart from the interventions being compared. Details of eight excluded studies, including reasons for exclusion, were presented. Reasons for exclusion included insufficient data and inadequate validity.

Specific interventions included in the review

The inclusion criteria were not specified in terms of the interventions, but it was clear that studies comparing any type of heated humidifier with any type of HME device were included. The included studies compared an extended-use hygroscopic condenser humidifier with heated water humidification (Fisher and Paykel), and a Pall BB-100 (hydrophobic/hydrophilic media bacterial/viral filter) that was changed every 24 hours with an unspecified type of heated water humidifier. Both of the included studies were conducted in the USA and both studies excluded high-risk patients.

Participants included in the review

Studies of intensive care unit (ICU) patients were eligible for inclusion. The stated aim of the review indicated that intubated ICU patients who had been ventilated for more than 48 hours were the focus of the review, while the search strategy implied that only adult patients (older than 18 years) were eligible. The included studies were conducted in medical, surgical and trauma ICU patients. The mean age of the participants ranged from 47 years for trauma patients to 58.4 years for medical and surgical ICU patients.

Outcomes assessed in the review

Studies that presented data on tracheal tube occlusion or VAP were eligible for inclusion. The diagnosis of VAP was made using objective predefined criteria in one included study, and criteria modified from the American College of Chest Physicians in the other study. Complete tracheal occlusion was diagnosed clinically in one study, and using a scoring system to assess the relative degree of obstruction (greater than 50%, or 25 to 50% of the internal diameter) on extubation or during bronchoscopy in the other study.

One of the included studies followed up patients until successful weaning from the ventilator, or hospital discharge or death, while the other study followed up patients for 6 months.

How were decisions on the relevance of primary studies made?

The author alone selected studies for inclusion.

Assessment of study quality

Only RCTs that met specified quality criteria (see Study Designs above) were included. Validity was also assessed based on criteria used to define the outcomes, adequacy of length of follow-up and source of funding. Validity was assessed, but the author did not state who performed the assessment.

Data extraction

The author did not state how the data were extracted for the review, or how many reviewers performed the data extraction. Where possible, data extracted included levels of statistical significance, confidence intervals (CIs) and relative risk (RR).

Methods of synthesis

How were the studies combined?

A narrative synthesis of the studies was undertaken.

How were differences between studies investigated?

Differences between the studies were discussed with reference to the type and age of the patients, interventions, outcomes and length of follow-up.

Results of the review

Two RCTs (602 patients) were included.

Both studies used adequate methods of randomisation, and in both studies the sample size was based on a power calculation. One RCT was partly funded by a respiratory equipment company.

VAP.

Both RCTs found that HME reduced VAP. One RCT found a statistically significant reduction in late onset hospital-acquired VAP (RR 0.38, CI not reported, P<0.05), but found no significant difference between treatments for early onset VAP (RR 1.1, CI and exact P-value were not reported). In the other RCT, the reduction was not statistically significant (RR 0.9, 95% CI: 0.46, 1.78).

Tracheal obstruction.

Neither RCT found any significant difference in tracheal obstruction between the types of humidification. No cases of tracheal obstruction were found in one RCT. The other RCT found the RR of greater than 50% occlusion was 0 and the RR of 25 to 50% occlusion was 1. Neither study calculated CIs or significance levels for tracheal obstruction.

Authors' conclusions

There was no difference in the incidence of tracheal tube occlusion between the types of humidification, but both identified studies found HME reduced VAP compared with heated water humidification. The author also concluded that the results may have limited generalisability since both studies were conducted in the USA and both excluded high-risk patients. Many studies were excluded because the information they presented was inadequate.

CRD commentary

The review question was clear in terms of the study design, intervention, participants and outcomes, although the inclusion criteria for the intervention were not explicitly stated. Several relevant sources were searched and the search terms were given. The author acknowledged that limiting the search to studies published in English that were available from specified sources might have resulted in the omission of other relevant studies. The author also acknowledged that the selection of studies by one person might have resulted in bias. The methods used to assess validity and extract the data were not described, but presumably these stages were also conducted by one person and the same limitations apply. Only RCTs that met certain validity criteria were included, although other aspects of validity were also addressed in the paper.

A narrative synthesis was appropriate given the identification of only two relevant RCTs. The author stated that this was a 'mini review' and as such involved no further calculations. Some of the limitations of the review were discussed: the possibility of publication bias; the review being conducted by only one person; and the exclusion of some studies because of the lack of information on the method of randomisation. The author stated that in both studies the sample size was based on a power calculation, but no details were provided of the outcomes on which this calculation was based. This omission means it cannot be determined whether the lack of statistical significance for VAP found by one RCT indicated a true lack of difference between the treatments, or whether this RCT was underpowered to detect a difference in VAP. Evidence in the review was limited to two RCTs comparing different devices, thus the conclusions should be interpreted with caution.

Implications of the review for practice and research

Practice: The author did not state any implications for practice.

Research: The author stated that further research on the effectiveness of HME in patients who are ventilated long-term is required. Such research should include different types of HME, should include high-risk patients, and should assess both relative and absolute tracheal obstruction.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.